An example of a conventional marine power distribution and propulsion system is shown in FIG. 1. A series of diesel engines D are used to drive individual generators G. These supply ac power to a first medium voltage switchboard or busbar MVAC1 and a second medium voltage switchboard or busbar MVAC2. The medium voltage busbars are equipped with protective switchgear that comprise circuit breakers and associated controls and are represented in FIG. 1 by the × symbol. Power converters PC are used to interface the medium voltage busbars to electric propulsion motors PM that drive a propeller. The medium voltage busbars are interconnected by protective switchgear.
Most of the ships services require low voltage and it is convenient to derive this from the medium voltage busbars by using a transformer. In the conventional marine power distribution and propulsion system shown in FIG. 1, a first low voltage switchboard or busbar LVAC1 is connected to the first medium voltage busbar MVAC1 through a first transformer T1 and protective switchgear. A second low voltage switchboard or busbar LVAC2 is connected to the second medium voltage busbar MVAC2 through a second transformer T2 and protective switchgear. The low voltage busbars are interconnected by protective switchgear. A series of unspecified electrical loads (labelled LVAC loads) can be connected to the low voltage busbars.
Although it is convenient to connect the low voltage busbars to the medium voltage busbars, such an arrangement normally leads to problematic coupling of harmonic distortion. In other words, harmonic distortion in the medium voltage busbars caused by the operation of the propulsion motors, for example, will be transferred through the transformers T1 and T2 to the low voltage busbars. If the electrical loads connected to the low voltage busbars require low harmonic distortion (i.e. a high quality of power supply (QPS)) then this may cause potential problems with their normal operation. Also, some of the loads connected to the low voltage busbars may themselves cause harmonic distortion in the low voltage busbars. This will be transferred through the transformers T1 and T2 to the medium voltage busbars.
In order to suppress such harmonic distortion, large and costly filters F are normally connected to the medium voltage busbars. Filters (not shown) may also be connected to the low voltage busbars.